1. Field of the Invention
The present invention relates to a wireless local area network (LAN) composed of plural access points, and more particularly to a handover processing required when a mobile station connected to the network moves between access points.
2. Description of the Background Art
Generally, a wireless LAN is composed of a plurality of basic service sets (BSS), a kind of wireless network, in each of which a base station, or access point (AP) works as a central coordinator adapted to communicate with a mobile terminal device, or station (STA), staying in the wireless network BSS. The access points are able to transfer communication frames to another station existing in the same wireless network BSS, or to transfer communication frames to another station existing in another wireless network BSS. In the latter case, transferring to a destination station existing in the other wireless network BSS, the communication frames sent from the originating station to the access point are further transferred to the other access point over a fixed communication channel interconnecting both of the access points, and then arrive at a destination station existing in the other wireless network BSS. In wireless LAN systems, an extended wireless network interconnecting the wireless networks or BSSs is called as an extended service set (ESS).
An extended wireless network ESS implements a service to interconnect wireless networks by connecting an access point to another access point, and also to connect a kind of network other than a wireless LAN. For example, an extended wireless network is connected to a wired LAN via a router or a switching device and further to the Internet over there. Thus, in the wireless LAN there are two kinds of service sets, the wireless network BSS which includes a central access point and plural stations, and the extended wireless network ESS which interconnects access points or are connected to a wired network other than a wireless LAN.
Communication frame formats transmitted over a wireless LAN include two types of frames such as a management and a data frame, for example. The management frame is used in transmitting a beacon signal periodically from an access point, and in transmitting various management information such as probe request, probe response, authentication, association and disassociation etc., between an access point and stations. The data frame is used in conveying data between a station accepted in joining a wireless network and an access point.
Each wireless network has its unique identification code BSSID to distinguish itself from other wireless networks in the wireless LAN, and sets the BSSID in the header of communication frames. A beacon frame and a data frame are transmitted at a common frequency under the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) system, and can be received by any stations in the wireless LAN.
When a moving station intends to communicate with a destination station over a wireless LAN, a procedure for joining and associating the wireless network will proceed. An access point, or base station, included in a wireless network transmits beacon frames indicating network information at constant time intervals in order to broadcast unified time information in its wireless service area and capability information required for mobile stations, or terminals, to join the wireless network. The beacon frame has a role to notify the entity of the wireless network to a mobile station approaching the wireless network. The station can select either to wait for a beacon frame reception or to send out a probe request frame to positively obtain network information from the access point. In the latter case, upon receipt of a probe request frame sent from the station, the access point shall send back a probe response frame. The probe response frame contains the same network information as the beacon frame does, so that the station can obtain the network information without waiting for a beacon frame.
When the station receives the beacon or probe response frame has been received, the station references information contained in the frame to determine whether or not the wireless network complies with its own de-sired condition, i.e. the station conforms to the wireless network to thereby decide to join the network. If the station intends to join, then it has to synchronize its own timer with the time information contained in the beacon or probe response frame of that intended wireless network.
The station thus joined next sends out an authentication request frame to the access point. When the access point receives the authentication request frame sent from the station, the access point determines whether or not to authenticate the station, and then sends out an authentication response frame containing a result from the authentication. If it is authenticated, the station proceeds to a next step to join. If not, however, the station has to give up to join the wireless network and withdraw from the network.
The station thus authenticated next sends out an association request frame to the access point. An association request frame contains information on the capability of the station. The access point, based on this information, determines whether or not the capability of the station is necessary and sufficient for the wireless network. When joining the network, the station determines whether or not its own condition complies with the condition of the wireless network to decide to join, as stated above. When associating the network, the access point evaluates the capability of the station. The result from the evaluation is contained in an association response frame, which will be sent out. If the station is to be associated, then it finishes the procedure. If not, however, the station has to give up to join the wireless network and withdraw from the network. The station that has completed the required joining and association procedures is now able to transmit and receive data frames.
The station thus approved for its association with the wireless network is now able to transmit and receive data frames. It also can receive beacon frames periodically transmitted from the access point. A beacon frame contains in its source address field SA the address of the access point and in its identification code field BSSID the identification code of wireless network, which is usually of the same level as the address of the access point. In addition, a beacon frame also contains time information and operation/management information required for the wireless network. The station receives beacon frames periodically to obtain and use the information to know the entity of the access point and the traffic conditions.
After a series of communication is finished, if the station associated with the wireless network now intends to withdraw from the wireless network, the station may send out a disassociation and a deauthentication frame to the access point to notify the latter of its withdrawal. From the access point, the associated station may receive a disassociation and a deauthentication frame to thereby be ordered to withdraw from the wireless network.
A mobile station, once associated with a wireless network and afterwards withdrew from the network, may rejoin the same or another wireless network within the same extended wireless network including the former wireless network by using a reassociation request frame. The reassociation procedure is to notify the address of the access point of the wireless network once associated to thereby simplify the procedure otherwise required for rejoining the same extended wireless network.
A handover function refers to the transfer of management of a mobile station, while moving, under the management of an access point to another access point. When a station already associated with a wireless network is going to make handover to a different wireless network, it will first withdraw from the wireless network including the access point currently associated, and next send a probe request frame to find out a new access point to which it approaches. The station receives a probe response frame sent from the new access point, and thereafter it carries out successive procedures, joining, authentication and reassociation with the new access point, thus completing its handover.
In recent years, the needs are increasing not only for the data communications but also for the voice communications using VoIP (Voice over Internet Protocol) technology, and a lot of terminal devices functioning as cellular phones are being marketed. Such terminal devices functioning as cellular phone are movable while included in communication with an access point, and therefore required to appropriately maintain continuous communication while moving from one access point to another to be connected.
However, a handover function implemented on such terminal devices serving as cellular phone associated with a wireless network would require a series of protocol processing, such as disassociation, joining, authentication and reassociation as described above, thus causing an interruption on communication over several hundred milliseconds. To data communications, not requiring a strict real-time transmission, the above mentioned procedures may be directly applied without difficulty. In applications such as VoIP or moving picture transmission, however, the interruption of transmission would cause a very serious problem. For example, in VoIP communications, voice frames have to be transmitted periodically every 50 millisecond, and therefore an interruption of communication over several hundred milliseconds seriously affects its communication quality.